FET-SEED integrated circuits offer considerable promise in optoelectronic processing, particularly for use in optoelectronic switches. SEEDs, because of the low energy requirements of their quantum well components, can receive optical inputs and provide optical outputs with a high degree of efficiency. They can be combined in simple circuits with minimal conductive paths, parasitic capacitance and line termination problems. For a survey of SEED devices and their applications, see D. A. B. Miller, "Quantum Well Optoelectronic Switching Devices," International Journal of High Speed Electronics, Vol. 1, No. 1, pp. 19-46 (1990).
One shortcoming of SEEDs for use in optoelectronic switching is the dependence of switching speed on the laser power level. In a large scale switching system the laser power provided to a large array of SEEDs may be subdivided by thousands in order to power each SEED in the array. Moreover, optical loss in spot generation further reduces the optical input power. Hence to achieve needed speeds in the range of hundreds of megabits per second, it is necessary to amplify the electrical signal from the quantum well receiving diode before applying the amplified signal to the input of the quantum well modulation diode.
While the FET is a good candidate for integration with quantum well diodes to form FET-SEED integrated circuits, integration which achieves high performance in both the FETs and the quantum well diodes without degrading the performance of one or the other must overcome a number of difficulties. For example, typical prior art quantum well diodes utilize a mesa structure presenting a difficult topography for defining FET gate regions with high precision. Moreover ideal FET channel layers often are highly optically absorbing and extension of such layers over optically sensitive diode regions can degrade diode performance. Furthermore FETs are voltage sensitive and the semi-insulating layers used in the fabrication of quantum well diodes permit the build up and lateral spread of spurious voltages which can degrade FET performance.
Contemplated electrooptic switching applications comprising large arrays of FETs and SEEDs require high performance components. The SEEDs must operate at hundred megabit speeds with low optical power, and the FETs must control source to drain voltages on the order of 10 V but must completely switch with a low magnitude threshold voltage on the order of -0.5 V in order to minimize the number of reference voltage leads.